1. Field of the Invention
The present invention relates to an anaerobic digestion process for organic wastes, particularly the anaerobic digest process which effectively copes with the widely fluctuating quantitative load of an organic waste slurry.
2. Description of the Prior Art
The anaerobic digestion process is known as one of the treating processes of such thick organic wastes as sewage treatment sludge, human/animal feces and alcohol distillery waste. This process transforms organic wastes into volatile fatty acids and then into methane, by the activities of the liquefying bacteria of the facultative anaerobic bacteria group, namely acidifying bacteria, and the gasifying bacteria of the obligatory anaerobic bacteria group. Furthermore, this process has characteristic features in that it has little possibility of spreading a secondary pollution and can be used as a driving power source of the facilities which treat the recovered methane gas. For these reasons, there is a move in recent years to employ it for treating not only water treatment sludge or human/animal feces but also wastes from various food processing industries.
In this concern, however, in many cases the volume of wastes discharged naturally varies widely during a day, a week or a year, so that in anticipation of such a wide fluctuation in the volume of wastes discharged, and in consideration of the varying volume of feed and impact of the scum generated, in most cases the digesters presently in operation are designed with a dimension for applying a safety coefficient around a double of the standard treatment volume of a digester. As the reaction of the anaerobic digestion process is slow, however, it is remarkably vulnerable to the impact of a sudden fluctuation in the feed load. In practice, in many cases the feed wastes are charged into the digester disregarding the above vulnerability, so that a significant number of digesters in this country are said to have been at a state of unsatisfactory operation. Therefore, in order to operate a digester while stably maintaining a high activity, the storing of feed organic wastes becomes imperative by all means. Conventionally, however, digesting the feed wastes after storing has been seen only in a few limited cases. In other words, a simple open-air storage permits generation of stink on account of decomposition and incurs a loss of chemical energy contained in the feedstock due to a biochemical oxidation by aerobic microorganism, which eventually invites a reduction in the methane recovery rate. These facts so far have discouraged storing the feed slurry. Also in a case of simple air-tight anaerobic storage, a partial gasification takes place in the course of storing whereby generating a gas of a lower methane concentration, which reduces the usable volume of recovered gas or results in a considerable amount of scum generation in the course of the subsequent digestion process.